1. Field of the Invention
The present invention relates to a process for producing dimethylnaphthalene (hereinafter sometimes abbreviated to "DMN"). More particularly, it pertains to a process for efficiently producing DMN with high yield and high selectivity over a long stabilized period by the dehydrogenation of dimethyl-tetrahydronaphthalene (hereinafter sometimes abbreviated to "DMTHN") under specific reaction conditions by the use of a specific catalyst.
DMN has numerous uses as a raw material for a high molecular material, such as a pharmaceutical material or the like. For example, 1,5-DMN which is obtained by the dehydrogenation of 1,5-DMTHN can be converted into industrially useful 2,6-naphthalene-dicarboxylic acid through the steps of isomerization and oxidation.
2. Description of the Related Arts
The process for dehydrogenating a tetrahydronaphthalene series compound into a naphthalene series compound in a liquid phase by the use of a noble metal catalyst supported on a powdery carrier has been known since long years before as a process for synthesizing a substituted naphthalene series compound.
For example, Eberhardt et al. discloses in J. Org. Chem., vol. 30 (1965), pp 82 to 84, a process for producing 1,5-, 1,7- or 1,4-DMN by dehydrogenating 1,5-, 1,7- or 1,4-DMTHN, respectively in liquid phase by the use of a palladium catalyst supported on a powdery activated carbon.
In addition, Japanese Patent Application Laid-Open No. 500052/1991 through PCT and U.S. Pat. No. 5,012,024 disclose the same process as above using a similar catalyst, except that the equilibrium conditions are brought to the advantageous side for DMN by continuously removing the hydrogen generated in the reaction system.
Moreover, U.S. Pat. No. 4,999,326 discloses the same process as above using a similar catalyst, which process includes a method of activating the catalyst deteriorated in reaction activity after reaction.
On the other hand, there is also known the process for dehydrogenating DMTHN into DMN in a gas phase as exemplified hereunder.
Japanese Patent Application Publication No. 30616/1975 and U.S. Pat. No. 3,781,375 disclose the use of a chromia/alumina catalyst in a gas-phase reaction.
In addition, Japanese Patent Application Laid-Open No. 67261/1973 discloses the use of rhenium or palladium catalyst supported on alumina in a gas-phase dehydrogenation of DMTHN.
Furthermore, Japanese Patent Application Publication No. 27694/1985 discloses a process for dehydrogenating DMTHN in a stream of hydrogen gas by the use of platinum catalyst supported on alumina.
Among the publicly known processes as mentioned above, the process for dehydrogenating DMTHN into DMN in a liquid phase by the use of a noble metal catalyst supported on a powdery carrier suffers the disadvantages that a long period is required for the completion of the reaction and the catalyst in fine powder form makes it extremely troublesome to separate the produced DMN from the catalyst. The process using a palladium catalyst supported on powdery activated carbon in the dehydrogenation in a liquid phase can not be said to be an industrially satisfactory process, since the process necessitates frequent regeneration of the catalyst repeatedly because of remarkable deterioration of catalyst activity as is revealed in U.S. Pat. No. 4,999,326.
On the other hand, the process for dehydrogenating DMTHN into DMN in a gas phase by the use of chromia/alumina catalyst or rhenium or palladium catalyst supported on alumina and the process for dehydrogenating DMTHN into DMN in a stream of hydrogen gas by the use of a platinum catalyst supported on alumina require a high reaction temperature in the range of 350.degree. to 450.degree. C. in order that a sufficient reaction rate may be maintained. Due to the high reaction temperatures, the above-mentioned processes suffer the drawbacks that the processes are liable to cause side reactions such as isomerization and demethylation of the methyl groups in DMN in the course of the reaction, and DMTHN and DMN tend to polymerize to form cokes on the catalyst, whereby the catalyst activity is subject to deterioration. Furthermore, the dehydrogenation of DMTHN into DMN is a marked endothermic reaction and therefore, the high reaction temperatures required in the aforesaid processes raised a serious problem regarding a heating system in the industrial reaction equipment. Specifically, it is impossible to adopt therein the heating system by the use of a multi-tube isothermal reactor using a heating medium oil which system is usually applied to an endothermic reaction at a relatively low temperature. Furthermore, an attempt to adopt therein an adiabatic reaction system that is widely employed for an endothermic reaction at a higher temperature necessitates the heating of DMTHN to a temperature much higher than the actually necessary reaction temperature to introduce it in the catalyst layer, which attempt unfavorably brings about remarkable side reactions such as isomerization and demethylation of the methyl groups in DMN.
As described hereinbefore, any of the aforestated conventional processes for dehydrogenating DMTHN into DMN can not render itself an industrially available process because of the serious defect remaining unsolved at the present time.